Signal filters are devices that process an input signal to provide a desired output signal. In the past, such filters have been based on analog components. Modern signal processing, however, has increasingly provided signals in digital form. In such an environment, a signal that is initially analog will be digitally sampled, and thereafter represented by the digital samples themselves. In order to process such signals, digital filters have been designed to operate directly on the digital samples.
A particular kind of digital filter is a comb filter. For a general discussion of comb filters, see Chu, Shuni et al., "Multirate Filter Designs Using Comb Filters," IEEE Transactions on Circuits and Systems, November 1984, pp. 913-924. As discussed therein, such filters have at least one differencer stage, at least one buffer stage, and at least one integrator stage, all connected in series.
As explained in Chu, during the normal operation of such filters, particularly the infinite impulse response ("IIR")-type, the possibility exists for data errors. The source of these errors is primarily noise. Once an error is introduced into the filter, the error will grow without bound, due to the recursive nature of the IIR algorithm. Ultimately, this will cause the stages to reach their maximum values and overflow. As a result, the filter will stop operating.
The foregoing problem becomes acute when such a filter is used in the transmission path of a typical digital radio. In such a radio, for instance, such a filter may be used to interpolate a digital signal prior to a final sigma-delta digital-to-analog ("D/A") converter used to drive a voltage-controlled oscillator ("VCO") in the transmission path. With this arrangement, an overflow of the filter will immediately block the transmission path, thus causing the radio to stop transmitting. The user of the radio experiencing this situation would find the radio transmission unintelligible.
To date there has been no satisfactory solution to this problem.